Delayed rectifier (DR) potassium channels are major determinants of cell membrane potential and function: the membrane potential of T-lymphocytes indirectly influences mitogenesis and cytokine secretion by affecting intracellular free calcium ion concentration; neuronal and muscle K channels modulate excitability and duration of action potentials; neurotransmitter release is also quite dependent upon the functional status of K channels at nerve terminals. T lymphocyte DR K channels modulating mitogenic and other inflammatory responses are of a relatively unique (Kv1.3) type lacking in electrically excitable cells, which possess mostly Kv1.1 and 1.2 types. This lymphocyte channel is the focus of considerable drug design effort, since specific blockers would be therapeutically useful in controlling several inflammatory diseases. Because of its homo-oligomeric structure, Kv1.3 also is an excellent model for investigating the molecular mechanisms by which selective peptide toxins interact with DR channels. We have shown that a sea anemone peptide called ShK toxin at subnanomolar concentrations selectively blocks Shaker type mammalian DR K channels, including Kv1.3. Our synthetic ShK toxin binds to lymphocyte Kv1.3 and rat brain K channels with potency equivalent to the natural toxin. ShK toxin contains approximately 31% helix, 18% B- sheet, and 17% B-turns. The toxin sequence lacks homology with scorpion K channel toxins and its disulfide pairings are also different. Preliminary analyses of synthetic ShK toxin analogs indicate that substitutions in two regions, 9-11 and 21-23, differently affect Kv1.3 and Kv1.2 K channels. We propose to map the ShK toxin molecular surface (pharmacophore) required for interaction with each of these twos channels, identify the major sites on the Kv1.3 K channel interacting with the toxin, and design smaller peptides and peptidomimetic compounds capable of selective Kv1.3 channel block. Kv1.3 blockers which do not affect other K channels should be useful in treating a variety of immune disorders.